1. Field of the Invention
The present invention relates generally to the field of coal pulverizers and, in particular, EL type pulverizers. More particularly, the present invention is drawn to several new features for EL type pulverizers which, when used either alone or in combination with each other, provide for longer wear life and reduced incidences of parts failures. Briefly, and as described in greater detail in the following description, the present invention includes: deep-dish contour grinding rings, tight tolerance ball tracks, heavy-duty top grinding ring flukes, larger diameter grinding balls, and an improved all-metal snubber design for limiting the horizontal movement of the top grinding ring.
2. Background of the Invention
Coal pulverizers are used to grind, dry and classify raw chunks of coal into fine solids which can be fluidized and fed, for example, to burners used in conjunction with industrial or utility boilers or furnaces. As is known to those skilled in the art, several different types of coal pulverizers, or coal mills, exist today, including one known by the designation “EL”.
EL type pulverizers were first produced in the early 1950's. EL type pulverizers are ball-and-ring (or ball-and-race) type pulverizers which employ the ball-bearing principle to grind the  coal. This design uses two vertical axis horizontal grinding rings, and a set of balls is placed between the grinding rings. The lower or bottom grinding ring rotates through connection to a rotating, vertical main shaft, while the upper or top grinding ring remains stationary and is spring loaded to create grinding pressure. The coal is ground by contact with the upper and lower grinding rings and balls (collectively, the grinding elements). The lower and upper grinding rings are each provided with a race having a predefined, matching track contour that engages the balls. The force from the upper grinding ring pushes the balls against the coal layer on the lower grinding ring. The grinding rings and the balls are made of abrasion resistant alloys and comprise the major wear parts of the mill. Ground coal is swept from the grinding zone defined by the grinding rings and the balls by air for final particle size classification and subsequent pneumatic transport to one or more coal burners. For further details of such EL type pulverizers, the reader is referred to Chapter 12 of Steam/its generation and use, 40th Edition, Stultz and Kitto, Eds., Copyright ©1992, The Babcock & Wilcox Company, the text of which is hereby incorporated by reference as though fully set forth herein.
EL mill top grinding rings have historically been loaded, and horizontal/rotational movement restricted, by using up to six single coil, dual-purpose springs. In a majority of cases, this design has proven to be an adequate means to restrict movement of the top grinding ring to allow operation without cyclic fatigue failures. However, when a coal with a high Hardgrove Grindability Index (HGI) is encountered, a high turndown is required from the mill, or when the mill is operated with little or no coal feed, the bed of pulverized coal on the lower grinding ring is severely reduced and metal-to-metal contact between the balls and grinding rings occurs. Such contact can cause excessive horizontal movement of the balls and top grinding ring due to undampened centrifugal forces. This excessive horizontal movement can be eccentric to the rotation of the lower grinding ring, resulting in premature wear and possible premature failure of the grinding rings, balls, springs and gearbox main shaft from cyclic metal fatigue.
Furthermore, such undampened centrifugal forces have allowed the balls to partially leave the grinding track established by the top and bottom grinding ring, since the previous design of the top and bottom grinding ring has not provided sufficient dampening forces required to counteract the centrifugal forces. This partial escape of the balls from their rotational track  has caused severe chipping of material off the top and bottom grinding ring's outside diameter, uneven wear patterns in the grinding rings, and spalling of metal off the ball surface. Any of these conditions have caused premature replacement of the grinding wear parts in the mill, along with additional internal components that may have failed indirectly as a result of the undampened centrifugal forces, such as the dual-purpose springs, housing units, and the gearbox main shaft.
Previous attempts to limit top grinding ring movement and subsequent failures of internal components include a device known as a snubber that is secured to an existing top grinding ring. Known snubbers are intended only to limit top grinding ring radial movement, and have been either bolted or welded to an existing top grinding ring. These snubbers experience failure because it is not possible to secure the snubber to the hard, high-chrome wear material of the grinding ring. Thus, catastrophic failures of grinding wear parts and other internal components will sometimes result from these attachment failures, in turn causing expensive and premature mill rebuilds and problems with the availability and reliability of the coal pulverizer.
Reliable coal pulverizer performance is essential for sustained full load operation of modern coal-fired electric power generation stations. An effective coal mill must be capable of handling a wide variety of coals and accommodating load swings without failure of internal parts.